When the /fill command is used to replace water with lava, it acts rather oddly. The bug only occurs when you actually use the "replace" argument, a normal fill command works fine.

Replacing lava with water doesn't cause this bug, but you can get an "unknown error" message if the selection was large enough.

To reproduce:

• Step 1: In a large body of water, use the following command:

  /fill ~-50 ~ ~-50 ~50 ~ ~50 lava 0 replace water

• Step 2: Be extremely confused.

Relates to MC-49545.